In LTE (Long Term Evolution) release 11, LTE_TDD_eIMTA (TDD: Time Divison Duplex; eIMTA: enhanced Interference Management Traffic Adaptation) is studied targeting on the efficient adaptation of TDD UL/DL (Uplink/Downlink) configuration to better match different UL/DL traffic load, in which UL-DL coexistence will be unavoidable, and UL-DL interference will be a big challenge.
Different cells may have different TDD UL/DL configurations depending on its UL/DL traffic load, so one UE (User Equipment) may experience quite different interference from neighbor cells in different TDD sub-frames. For example, in UL direction, one UE may suffer low inter-cell interference in one sub-frame from UEs (UL to UL interference case) in the neighbor cells, but may suffer quite high inter-cell interference (DL to UL interference case) from a neighbour eNB in another sub-frame.
For TDD LTE, fixed HARQ (Hybrid Automatic Repeat Request) timeline has been specified for different TDD UL/DL configurations depending on the specific TDD frame structures. An example for a DSUUU configuration (uplink-downlink configuration 0 in FIG. 4) is shown in FIG. 1. In FIG. 1, U denotes a subframe for UL transmission, D denotes a subframe for DL transmission and S denotes a special subframe comprising a switch-point for changing the transmission direction.
When considering the UL HARQ process for LTE_TDD_eIMTA, if the traditional HARQ timeline is used for the UL-DL coexistence scenarios, one problem is that the new transmission packet and the retransmission packet may have a quite different SINR (Signal to Interference plus Noise Ratio). This is because HARQ timeline for UL in LTE is synchronous and anew transmission packet associated with HARQ ID 1 may be transmitted in UL in the pico cell for instance in subframe 2 in parallel with UL data in the macro cell, while the next retransmission for HARQ ID 1 in UL in the pico cell takes place in subframe 3 in parallel with DL data transmission in the macro cell, as shown in FIG. 2.
Usually, a retransmission has the same MCS (Modulation and Coding Scheme) as a new transmission (such as chase combining), or similar MCS as a new transmission (such as IR (Incremental Redundancy) method). Hence, if the SINR difference between transmission and retransmission is too high, there will be either unnecessary energy redundancy when the retransmission has a much higher SINR than the transmission or there will be a helpless retransmission when the retransmission has much lower SINR than the transmission and it is hard to recover the signals.
One common understanding in current eIMTA studies is that subframe #0/1/2 and subframe #5/6/7 are fixed as D-S-U to protect transmission of important control signaling, at least for a 5 ms UL-DL switching period. For protected sub-frames it is ensured that all cells in the network configure the same type D, S or U for data transmission carried out in a protected sub-frame. The first transmission for HARQ ID 1 in the pico cell in the scenario depicted in FIG. 2 will therefore experience interference only from UL transmissions in neighboring cells.
However, one should note that this agreement is not sufficient for protecting the retransmissions for HARQ ID 1 in the pico cell in the scenario of FIG. 2. Although the first transmission occurs in the protected sub-frame 2 the second (re)transmission takes place in the unprotected sub-frame 3 in parallel with DL data transmission in the macro cell, even though both packets are related to the same control signaling.